The invention relates to active filtering. It is known to use active filters in association with a power source in order to eliminate ripple, or distortion, due to the source, or to a load across the source.
It is known from United States Pat. No. 3,648,150 of Andress Kernick et al to control an inverter through a "bang-bang" feedback loop including an L-C filter in order to create a switching-type quasi-linear amplifier between a power source and a load. Such time-optimal-response feedback reduces the ripple, or distortion, across the load by combining the two functions of regulated DC-AC conversion and active filtering. The filtering technique used in the Kernick patent consists in providing source-directing and fast-acting switches that synthesize a distortion-free waveform of desired magnitude reproducing a reference signal through an anticipatory control system which generates a ripple-free output despite disturbances arising from either the power source or the load. However, the switching speed of relatively large powder switches in a converter may be a limiting factor. Therefore, it has been suggested in U.S. Pat. No. 3,825,815 of Laszlo Gyugyi et al to separately perform the filtering function by rejecting the ripple through an auxiliary but fast-switching converter in parallel, or in series, with the power lines, or the main converter having such larger but slower switching devices. In the auxiliary converter, or active filter, controlled surges of energy are alternately stored and released so as to actually generate a ripple component of same magnitude and frequency as the undesired one in the main lines. The active filter in fact injects such created ripple in phase opposition to the source, or load ripple, so as to neutralize the two effects. The energy storage element in the active filter may be an inductor, or a capacitor.
In active filtering, the average energy stored in the inductor, or capacitor, has to be sustained by a control action capable of anticipating the losses which in the storage device are to be replenished. Therefore, with the Kernick patent where active filtering is directly in the power lines, since the power converter is its own active filter, anticipatory control for the "bang-bang" feedback loop is important in containing any discrepancy of the L-C output filter within the two limits of the deadband. In contrast, with the Gyugyi patent "bang-bang" switching is instantaneous. Therefore, where there is separate and auxiliary active filtering, anticipatory control would be superfluous.
Another particularity of the prior art technique of active filtering described in the aforementioned Kernick and Gyugyic patents is that the power converter, there, is inserted between an alternating current power source and a direct current load, or between an alternating current power source and an alternating current load.
In contrast, the object of the present invention is to link a direct current power source, or a DC to DC power converter, to a direct current load while at the same time providing:
(1) active filtering; (2) a DC energy pump of controlled output; (3) voltage adjustment and/or (4) isolated terminals, with a minimum number of components.